As it is known in the telecommunications art, dense wavelength division multiplexing (DWDM) is a technology that allows many wavelengths of light to travel along the same fiberoptic cable. Each of those wavelengths of light convey a stream of data that is filtered and decoded when it reaches an optical DWDM receiver. DWDM technology significantly increases the amount of data that can be conveyed, at a single time, across a fiberoptic cable and hence is becoming highly utilized.
DWDM systems typically include equipment to monitor the optical signals communicated thereon to determine, among other characteristics, their wavelength and intensity. Such monitors are typically referred to as optical spectrum analyzers. Fixed and tunable Fabry-Perot interferometers, characterized by narrow passband periodic transmission functions, are used as wavelength etalons in these optical spectrum analyzers. Such interferometers are also utilized in wavelength tunable lasers that drive the DWDM signals.
There are two widely used measurement schemes, involving Fabry-Perot cavities, referred to as the static scheme and the dynamic scheme. With respect to the static scheme, broadband or laser light transmitted through a fixed Fabry-Perot cavity forms an interference pattern. The wavelength and intensity of light under test are judged by the form and shape of the resultant optical signal, taking into account fixed transmission characteristic of the cavity. In the dynamic scheme, the optical path in the Fabry-Perot cavity is dynamically changed by adjusting the incidence angle of light or by adjusting the spacing between the mirrors. In the dynamic scheme, the input light forms a dynamically changing interference pattern and the properties of the light are judged by the resultant dynamic optical signal.
Both methods require prior determination of the transmission function of the Fabry-Perot interferometer being utilized. The accuracy of the measurement is dependent upon how precisely that transmission function is known and how stable it is. The stability of the transmission function is highly dependent on the physical characteristics of the Fabry-Perot cavity. These physical characteristics are highly susceptible to environmental changes and, therefore, the accuracy of the above-mentioned methods are significantly dependent thereon.
Accordingly, an algorithmic mechanism is needed for providing accurate and reliable measurements across wide operating and environmental ranges.
In accordance with an aspect of the present invention, an algorithmic method and apparatus is provided for improving the accuracy of Fabry-Perot based measurement systems by performing the calibration of the cavity transmission function and introducing the correction into the measurement results.
In one embodiment of the present invention, a method for calibrating an etalon based optical system includes the steps of applying an optical signal having a known spectral shape to the etalon. The resulting transmission peaks are recorded and analyzed at selected points, in the wavelength domain. Accordingly, the centre wavelength, peak transmission and full-width-half-max parameters are determined. The center wavelength, peak transmission and full-width-half-max parameters are subsequently applied to a formula that approximates the response of the etalon such that the etalon is calibrated.
In a further embodiment of the present invention, the peak transmission is determined by performing a least square fit of the shape of the resulting transmission peaks to the aforementioned formula.